Semiconductor wafers or other such substrates are subjected to very high processing temperatures. For example, in chemical vapor deposition (CVD), the temperatures approach 1200.degree. C. In a typical cycle, a wafer is transferred from a room temperature cassette by a robotic wafer handler into a reactor chamber where it is subjected to the high temperature processing and is then transferred by the wafer handler from the high temperature chamber back to the same cassette or a separate cassette for processed wafers. Because of the high temperature CVD processing, transport from the process chamber directly to a wafer cassette is not possible due to the temperature of the wafer exceeding the material properties of most commonly used cassette materials. Because of this, the transfer of the wafer to a cassette must be postponed until the wafer temperature falls below the thermal properties of the cassette material. While cassettes are available that can handle wafers as hot as 170.degree. C., they are relatively expensive. A commonly available less expensive one made of Delrin.RTM. can only handle temperatures well below 100.degree. C. Other commonly available units can only handle about 60.degree. C. Hence, it is a desirable goal that the temperature of a wafer be quickly cooled to that level.
Because the wafer handling and processing occurs in an enclosed and carefully controlled environment, there are essentially only three locations or points during the cycle where the cooling of the wafer might occur. The wafer could be cooled on the susceptor on which it is supported in the process chamber, on the wafer handling device, or off-line at some location within the apparatus. Cooling the wafer on the susceptor is not cost-effective because the process chamber is then unavailable for processing another wafer, thereby reducing the system wafer throughput. This approach is particularly unattractive because it is then necessary to incur the delay and cost of reheating the chamber. Removing a wafer while it is hot and cooling it on the wafer handling device is better, but also not cost effective because the delay in loading the next wafer slated for processing compromises throughput. Such impediments increase the per-wafer cost, making these approaches financially unattractive to end users. Because of the high cost of semiconductor wafer processing equipment, it is, of course, critically important from a competitive standpoint to be able to keep the expensive equipment in continued use so as to increase the throughput. At the same time, the wafer cooling technique employed must be compatible with the environment of the CVD processing apparatus so as not to adversely affect stringent cleanliness requirements. Also, the cost of the technique must itself be sufficiently moderate so that there is a net reduction in the per-wafer cost.
Accordingly, it is an object of this invention to provide an improved system for quickly cooling wafer-like substrates to a temperature that will allow the use of low cost commonly available cassettes.